U.S. patent application number 12/411617 was filed with the patent office on 2009-10-01 for image forming apparatus.
This patent application is currently assigned to KYOCERA MITA CORPORATION. Invention is credited to Tamami Asari, Syoukou Gon, Naoyuki Ishida, Kenichi Kasama, Akihirio Kondo, Eiji Nakajima, Yuzuru Nanjo.
Application Number | 20090245898 12/411617 |
Document ID | / |
Family ID | 41117468 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090245898 |
Kind Code |
A1 |
Gon; Syoukou ; et
al. |
October 1, 2009 |
IMAGE FORMING APPARATUS
Abstract
An image forming apparatus includes a fixing unit fixing a toner
image onto a sheet. The fixing unit includes a heating member
having a sheet conveyed region that is set in accordance with the
size of the sheet. The fixing unit further includes a coil
generating magnetic field, a fixed core forming a magnetic path,
movable cores forming a magnetic path together with the fixed core
and arranged along the sheet conveyed region, a shielding member
arranged on at least one movable core and shielding magnetism, and
a magnetism adjustment unit rotating at least one movable core to
switch the position of the shielding member between a shielding
position where the shielding member is positioned inside the sheet
conveyed region to shield the magnetism and a retracted position
where the shielding member is positioned outside the sheet conveyed
region to permit pass of the magnetism.
Inventors: |
Gon; Syoukou; (Osaka-shi,
JP) ; Nanjo; Yuzuru; (Osaka-shi, JP) ; Kondo;
Akihirio; (Osaka-shi, JP) ; Nakajima; Eiji;
(Osaka-shi, JP) ; Ishida; Naoyuki; (Osaka-shi,
JP) ; Kasama; Kenichi; (Osaka-shi, JP) ;
Asari; Tamami; (Osaka-shi, JP) |
Correspondence
Address: |
CASELLA & HESPOS
274 MADISON AVENUE
NEW YORK
NY
10016
US
|
Assignee: |
KYOCERA MITA CORPORATION
Osaka-shi
JP
|
Family ID: |
41117468 |
Appl. No.: |
12/411617 |
Filed: |
March 26, 2009 |
Current U.S.
Class: |
399/328 ;
399/329 |
Current CPC
Class: |
G03G 2215/2032 20130101;
G03G 15/2042 20130101 |
Class at
Publication: |
399/328 ;
399/329 |
International
Class: |
G03G 15/20 20060101
G03G015/20 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2008 |
JP |
2008-085377 |
Jun 30, 2008 |
JP |
2008-170520 |
Claims
1. An image forming apparatus comprising: an image forming section
forming a toner image and transferring the toner image onto a
sheet; and a fixing unit including a heating member and a pressure
member, the fixing unit operable to fix the toner image onto the
sheet while nipping and conveying the sheet between the heating
member and the pressure member, wherein: the heating member has a
sheet conveyed region that the sheet passes, the sheet conveyed
region being set in accordance with the size of the sheet being
conveyed, and the fixing unit further includes: a coil arranged
along an outer surface of the heating member and generating a
magnetic field, a fixed core arranged opposite to the heating
member with respect to the coil and forming a magnetic path, a
plurality of movable cores arranged between the fixed core and the
heating member with respect to a direction in which the coil
generates a magnetic field, to form the magnetic path together with
the fixed core, and also arranged along the sheet conveyed region,
a shielding member arranged along an outer surface of at least one
movable core and shielding magnetism, and a magnetism adjustment
unit rotating at least one movable core around a predetermined axis
to switch the position of the shielding member between a shielding
position where the shielding member is positioned inside the sheet
conveyed region to shield the magnetism and a retracted position
where the shielding member is positioned outside the sheet conveyed
region to permit pass of the magnetism.
2. The image forming apparatus according to claim 1, wherein: the
shielding member is provided on the outer surface of each movable
core; and the magnetism adjustment unit rotates the plurality of
movable cores individually.
3. The image forming apparatus according to claim 2, wherein the
magnetism adjustment unit includes: a common rotation unit
simultaneously rotating the outer movable cores arranged at
positions corresponding to ends of a maximum sheet conveyed region
set when a sheet having a maximum size is conveyed; and a plurality
of individual rotation units individually rotating a corresponding
one of the other inner movable cores positioned between the outer
movable cores.
4. The image forming apparatus according to claim 2, wherein: the
movable cores include a first movable core arranged inside a
minimum sheet conveyed region set when a sheet having a minimum
size is conveyed, and a second movable core arranged outside the
minimum sheet conveyed region; and the shielding member is provided
in not the first movable core but the second movable core.
5. The image forming apparatus according to claim 3, wherein: the
outer movable core and the inner movable core are each a
cylindrical core having a through hole formed along the axis
thereof; the common rotation unit includes a rotating shaft member
fitted in the through holes of the outer movable cores and fitted
loosely in the through holes of the inner movable cores, and a
drive source rotating the rotating shaft member; and each of the
individual rotation units includes a rotating roller pressed into
contact with an peripheral surface of the corresponding inner
movable core and undergoing rotation to transmit a friction force
to the peripheral surface, and a drive source rotating the rotating
roller.
6. The image forming apparatus according to claim 1, wherein among
the plurality of movable cores, the magnetism adjustment unit
rotates a movable core arranged outside the sheet conveyed region
set in accordance with the size of the sheet to switch the position
of the shielding member of the movable core from the retracted
position to the shielding position.
7. The image forming apparatus according to claim 6, wherein: the
plurality of movable cores are formed by dividing a single core
into a plurality of cores, the single core having a through hole of
a circular sectional shape formed along the axis thereof; the
magnetism adjustment unit includes a shaft member fitted loosely in
the through holes of the movable cores and supporting the movable
cores rotatably, a guide groove formed in an inner peripheral
surface of each movable core, an engagement portion provided in the
shaft member and engageable with the guide groove, and a drive
mechanism driving the shaft member; and the shape of the guide
groove is set in such a way that as the shaft member is driven, the
engagement portion moves in the guide groove to rotate the movable
cores.
8. The image forming apparatus according to claim 7, wherein: the
engagement portion is a plurality of projections provided on an
outer peripheral surface of the shaft member and spaced at a
predetermined interval from each other in the axial direction of
the shaft member; the drive mechanism includes a moving mechanism
moving the shaft member in the through holes in the axial direction
of the movable cores and a rotation mechanism rotating the shaft
member in the through holes around the axis of the shaft member;
the guide groove includes an axial groove formed at the inner
peripheral surfaces of the movable cores over the movable cores in
the axial direction of the movable cores, and a circumferential
groove formed at the inner peripheral surface to extend from the
axial groove in the circumferential direction of the movable core;
the axial groove has a shape capable of receiving the projections,
the projections moving in the axial groove relative to the movable
cores in the axial direction of the movable cores when the moving
mechanism moves the shaft member; the circumferential groove has a
shape capable of receiving the projections, the projections moving
in the circumferential groove relative to the movable cores in the
circumferential direction of the movable core when the rotation
mechanism rotates the shaft member; when the moving mechanism moves
the shaft member in the axial direction, the projections are
switched to a position where the projections are received in the
circumferential groove or a position where the projections are not
received in the circumferential groove; and when the projections
are switched to the position where the projections are received,
the rotation of the shaft member by the rotation mechanism keeps
the shielding member in the retracted position, while when the
projections are switched to the position where the projections are
not received, the rotation of the shaft member by the rotation
mechanism switches the position of the shielding member from the
retracted position to the shielding position.
9. The image forming apparatus according to claim 8, wherein: the
movable core is a cylindrical core and, instead of the shielding
member, includes a cut-out portion so formed by cutting off a
peripheral part thereof as to have an arcuate shape in section
viewed from the axial direction; and when the projections are
switched to the position where the projections are received in the
circumferential groove, the rotation of the shaft member by the
rotation mechanism keeps the cut-out portion in the retracted
position, while when the projections are switched to the position
where the projections are not received in the circumferential
groove, the rotation of the shaft member by the rotation mechanism
switches the cut-out portion from the retracted position to the
shielding position.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an image forming apparatus
including a fixing unit which fixes a heated and melted unfixed
toner onto a sheet carrying a toner image while passing the sheet
of paper through a nip defined between a pair of heating rollers or
a heating belt and a roller.
[0003] 2. Description of the Related Art
[0004] In this type of image forming apparatus, in order to meet
demands such as shortening the warm-up time of a fixing unit and
saving energy, attention has recently been drawn to a belt method
capable of operating with a smaller amount of heat capacity (e.g.,
refer to Japanese Patent Laid-Open Publication No. H06-318001). In
recent years, an electromagnetic induction heating method (IH)
capable of rapid heating or high-efficient heating has also been
notable, and taking into account saving energy when fixing a color
image, the image forming apparatus employing the combination of the
electromagnetic induction heating and belt methods have been put on
the market. The combination of the belt method and the
electro-magnetic induction heating has advantages in that a coil
can be easily laid out and cooled and a belt can be directly
heated. These and other advantages prompt an electromagnetic
inductor to be arranged outside of the belt (so-called external IH
type)
[0005] In the electro-magnetic induction heating method, various
arts have been developed for the purpose of preventing an excessive
temperature rise in a non-sheet conveyed region in accordance with
the width (conveyed-sheet width) of a sheet conveyed through a
fixing unit. Particularly, a means for the different sizes of
sheets in the external IH is described in the following prior arts,
for example, Japanese Patent Laid-Open Publication No. 2003-107941
and Japanese Patent Laid-Open Publication No. 2006-120523).
[0006] In a first prior art (Japanese Patent Laid-Open Publication
No. 2003-107941 (FIGS. 2 and 3)), a magnetic member is divided into
several parts and arranged in a conveyed-sheet width direction, and
some of the divided parts of the magnetic member are moved close to
and apart from an excitation coil in accordance with the width
(conveyed-sheet width) of a conveyed sheet. In this case, some of
the divided parts of the magnetic member are moved apart from the
excitation coil in a non-sheet conveyed region, thereby lowering
the heat-generation efficiency in the non-sheet conveyed region to
make the generated-heat quantity smaller than that in a minimum
sheet conveyed region for a sheet of a minimum width.
[0007] In a second prior art (Japanese Patent Laid-Open Publication
No. 2006-120523), a magnetic shielding plate having a
curved-surface is formed in advance with a plurality of steps in
the longitudinal directions thereof, and these steps form an area
for passing magnetism and an area screening out magnetism in the
width direction of a sheet. Therefore, when the size of a sheet is
changed, the magnetic shielding plate is turned in accordance with
the conveyed-sheet width, thereby screening out magnetism in a
non-sheet conveyed region to suppress an excessive rise in the
temperature of a heated roller or the like.
[0008] However, the first prior art has the problem of requiring a
wider motion space for the magnetic member, thereby making the
whole apparatus larger.
[0009] In the second prior art, the positions of the steps formed
beforehand in the shielding plate determine the shielding area and
the non-shielding area, thereby making it difficult to handle
sheets of paper having many different sizes. Besides, if the steps
are formed in the direction in which the shielding plate turns,
then the turning angle as a whole is restricted to hinder enlarging
each step (e.g., a turning angle of approximately
15.degree.-30.degree.), thereby reducing the quantity of
screened-out magnetism and making it impossible to suppress the
generated-heat quantity sufficiently.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide an image
forming apparatus capable of decreasing the number of members
arranged inside a heating member to reduce the heat capacity,
shorten the warm-up time and save a space, and also capable of
regulating magnetism for a variety of sheet sizes and producing a
shielding effect enough.
[0011] In order to accomplish the object, an image forming
apparatus according to the present invention includes an image
forming section forming a toner image and transferring the toner
image onto a sheet and a fixing unit including a heating member and
a pressure member. The fixing unit is operable to fix the toner
image onto the sheet while nipping and conveying the sheet between
the heating member and the pressure member. The heating member has
a sheet conveyed region that the sheet passes. The sheet conveyed
region is set in accordance with the size of the sheet being
conveyed. The fixing unit further includes a coil arranged along an
outer surface of the heating member and generating a magnetic
field, a fixed core arranged opposite to the heating member with
respect to the coil and forming a magnetic path, a plurality of
movable cores arranged between the fixed core and the heating
member with respect to a direction in which the coil generates a
magnetic field, to form the magnetic path together with the fixed
core, and also arranged along the sheet conveyed region, a
shielding member arranged along an outer surface of at least one
movable core and shielding magnetism, and a magnetism adjustment
unit rotating at least one movable core around a predetermined axis
to switch the position of the shielding member between a shielding
position where the shielding member is positioned inside the sheet
conveyed region to shield the magnetism and a retracted position
where the shielding member is positioned outside the sheet conveyed
region to permit pass of the magnetism.
[0012] These and other objects, features and advantages of the
present invention will become more apparent upon reading of the
following detailed description along with the accompanied
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic view showing a configuration of an
image forming apparatus according to an embodiment of the present
invention.
[0014] FIG. 2 is a longitudinal sectional view showing a structural
example of a fixing unit.
[0015] FIG. 3 is a plan view showing in detail a configuration of a
center core divided in the axial direction.
[0016] FIGS. 4A and 4B are longitudinal sectional views showing an
operation as a block-shaped core rotates.
[0017] FIGS. 5A and 5B are a side view showing an end part of the
center core and a partial sectional view (longitudinal section
along a B-B line) showing an operation thereof, respectively.
[0018] FIGS. 6A and 6B show a control example in accordance with
each of a minimum conveyed-sheet width and a maximum conveyed-sheet
width.
[0019] FIGS. 7A to 7G show a control example for an intermediate
conveyed-sheet width.
[0020] FIGS. 8A to 8G show a control example for a maximum
conveyed-sheet width.
[0021] FIG. 9 is a longitudinal sectional view showing a further
structural example of the fixing unit.
[0022] FIG. 10 is a longitudinal sectional view showing a still
further structural example of an IH coil unit.
[0023] FIG. 11 is a longitudinal sectional view showing a
structural example of a fixing unit.
[0024] FIGS. 12A to 12E are plan views showing in detail a
configuration of a center core divided in the axial direction.
[0025] FIGS. 13A to 13D are vertical sectional views showing a
rotation or non-rotation state of a block-shaped core as a shaft
member rotates.
[0026] FIG. 14 is a side view showing a configuration of a rotation
mechanism and a moving mechanism of the shaft member.
[0027] FIG. 15 is a side view showing the configuration of the
rotation mechanism and the moving mechanism of the shaft
member.
[0028] FIGS. 16A and 16B are longitudinal sectional views showing
an operation as the shaft member rotates.
[0029] FIG. 17 is a longitudinal sectional view showing a further
structural example (second example) of the fixing unit.
[0030] FIG. 18 is a longitudinal sectional view showing a still
further structural example (third example) of the fixing unit.
[0031] FIG. 19 is a longitudinal sectional view showing a still
further structural example (fourth example) of the fixing unit.
[0032] FIG. 20 is a longitudinal sectional view showing a further
structural example of an IH coil unit.
[0033] FIG. 21 is a longitudinal sectional view showing a
structural example of an internal type IH coil unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] Embodiments of the present invention will be described below
in detail with reference to the drawings.
[0035] FIG. 1 is a schematic view showing a configuration of an
image forming apparatus 1 according to an embodiment of the present
invention. The image forming apparatus 1 conducts printing by
transferring a toner image onto a surface of a printing medium such
as printing paper according to image information and is, for
example, a printer, a copying machine, a facsimile device, or a
complex machine having some of the functions thereof.
[0036] The image forming apparatus 1 of FIG. 1 is a tandem-type
color printer and includes an apparatus main body 2 shaped like a
rectangular-parallelepiped box which forms (prints) a color image
on a sheet inside thereof. The apparatus main body 2 is provided on
the top with a paper discharge portion (discharge tray) 3 on which
a sheet after a color image is printed thereon is discharged.
[0037] The apparatus main body 2 houses a paper cassette 5 storing
sheets in a lower part thereof and is provided on a side (the right
side in FIG. 1) with a stack tray 6 for manual feeding. Above the
paper cassette 5, the apparatus main body 2 houses an image forming
section 7 forming an image on a sheet based upon image data such as
characters and pictures transmitted from the outside.
[0038] The apparatus main body 2 is provided inside at a left
portion in FIG. 1 with a first conveying path 9 conveying to the
image forming section 7 a sheet delivered from the paper cassette
5. Formed between the stack tray 6 and the first conveying path 9
is a second conveying path 10 which conveys a sheet delivered from
the stack tray 6 to the image forming section 7. The apparatus main
body 2 is provided inside at an upper-left portion with a fixing
unit 14 which gives fixing to a sheet on which an image is formed
in the image forming section 7, and a third conveying path 11
conveying a sheet to the paper discharge portion 3 after the
fixing.
[0039] The paper cassette 5 can be drawn out of the apparatus main
body 2 and then refilled with sheets, and includes a storage
portion 16 selectively storing at least two kinds of sheets having
different sizes. Sheets of paper stored in the storage portion 16
are delivered one by one to the first conveying path 9 by a sheet
feeding roller 17 and a handling roller 18.
[0040] The stack tray 6 can be opened and closed on the right side
of the apparatus main body 2 and includes a manual feeding portion
19 on which a single or a plurality of sheets are placed manually.
Sheets placed on the manual feeding portion 19 are delivered one by
one toward the second conveying path 10 by a pick-up roller 20 and
a handling roller 21.
[0041] The first conveying path 9 and the second conveying path 10
join in front of a resist roller 22. A sheet supplied to the resist
roller 22 waits once here, is sent to a secondary transfer portion
23 after undergoing a skew adjustment and a timing adjustment, and
is given a secondary transfer of a full-color toner image on an
intermediate transfer belt 40 in the secondary transfer portion 23.
The sheet subjected to toner-image fixing in the fixing unit 14 is
reversed, if necessary, in a fourth conveying path 12 and the side
of the sheet reverse to the side subjected to the toner-image
fixing undergoes the secondary transfer of a full-color toner image
in the secondary transfer portion 23. After undergoing the
toner-image fixing on the reverse side in the fixing unit 14, the
sheet passes through the third conveying path 11 and is discharged
to the paper discharge portion 3 by a discharge roller 24.
[0042] The image forming section 7 includes four image forming
units 26 to 29 forming each toner image of black (B), yellow (Y),
cyan (C) and magenta (M), and an intermediate transfer unit 30
superimposing and carrying toner images of each color formed by the
image forming units 26 to 29.
[0043] Each of the image forming units 26 to 29 includes a
photosensitive drum 32 rotating counterclockwise as shown by an
arrow by a drive motor (not shown), a charger 33 mounted face to
face with a peripheral surface of the photosensitive drum 32, a
laser scanning unit 34 arranged downstream of the charger 33 in the
rotational direction of the photosensitive drum 32 and applying a
laser beam to a specified position on the peripheral surface of the
photosensitive drum 32, a developer 35 arranged downstream of the
laser-beam radiation position in the rotational direction of the
photosensitive drum 32 and mounted face to face with the peripheral
surface of the photosensitive drum 32, and a cleaner 36 arranged
downstream from the developer 35 in the rotational direction of the
photosensitive drum 32 and mounted face to face with the peripheral
surface of the photosensitive drum 32.
[0044] Each developer 35 of the image forming units 26 to 29
includes a toner box 51 storing each of a black toner, a yellow
toner, a cyan toner and a magenta toner.
[0045] The intermediate transfer unit 30 includes a rear roller
(driving roller) 38 mounted near the image forming unit 26, a front
roller (driven roller) 39 mounted near the image forming unit 29,
the intermediate transfer belt 40 stretched between the rear roller
38 and the front roller 39, and four transfer rollers 41 pressed
via the intermediate transfer belt 40 against the peripheral
surface of the photosensitive drum 32 of each image forming unit 26
to 29.
[0046] In the intermediate transfer unit 30, toner images of each
color are superimposed and transferred at the positions of the
transfer rollers 41 from the photosensitive drums 32 onto the
intermediate transfer belt 40, and a full-color toner image is
formed on the intermediate transfer belt 40.
[0047] The first conveying path 9 conveys a sheet delivered from
the paper cassette 5 to the intermediate transfer unit 30 and
includes a plurality of conveying rollers 43 arranged in
predetermined positions, and the resist roller 22 arranged in front
of the intermediate transfer unit 30 and adjusting the timing
between an image forming operation by the image forming section 7
and a paper feeding operation.
[0048] The fixing unit 14 heats and pressurizes a sheet, on which a
toner image is transferred in the image forming section 7, to fix a
toner image on the sheet. The fixing unit 14 includes, for example,
a roller pair made up of a pressure roller 44 and a fixing roller
45 of a heating type. The pressure roller 44 has, for example, a
metal core and an elastic surface layer (e.g., silicone rubber),
and the fixing roller 45 has, for example, a metal core, an elastic
surface layer (e.g., silicone sponge) and a mold-release layer
(e.g., PFA). A heat roller 46 is arranged adjacent to the fixing
roller 45, and a heating belt 48 is stretched between the heat
roller 46 and the fixing roller 45. A specific structure of the
fixing unit 14 will be further described later.
[0049] A conveying path 47 is formed on each of the upstream and
downstream sides of the fixing unit 14 in the sheet conveying
direction. Through the upstream conveying path 47, a sheet passed
through the intermediate transfer unit 30 is introduced into the
nip between the pressure roller 44 and the fixing roller 45, and
through the nip, is guided to the third conveying path 11 via the
downstream conveying path 47.
[0050] The third conveying path 11 forwards a sheet subjected to
fixing in the fixing unit 14 to the paper discharge portion 3, and
is provided in a proper position with a conveying roller pair 49
and at the outlet with the discharge roller 24.
First Embodiment
[0051] [Details of Fixing Unit]
[0052] Next, the fixing unit 14 of the image forming apparatus 1
according to a first embodiment of the present invention will be
described in detail.
[0053] FIG. 2 is a longitudinal sectional view showing a structural
example of the fixing unit 14. In FIG. 2, the fixing unit 14 is
shown with turned counterclockwise by approximately 90 degrees from
a state thereof mounted in the image forming apparatus 1, and
hence, the sheet conveying direction extends from right to left,
though from below to above in FIG. 1. If the apparatus main body 2
is relatively large (complex machine or the like), the fixing unit
14 can be mounted in the direction given in FIG. 2, and in addition
to the above, the fixing unit 14 may be arranged with inclined
laterally from the state of FIG. 2.
[0054] As described above, the fixing unit 14 includes the pressure
roller 44, the fixing roller 45, the heat roller 46 and the heating
belt 48. The surface layer of the fixing roller 45 is formed with
the elastic silicone sponge layer to form a flat nip between the
heating belt 48 and the fixing roller 45.
[0055] The heating belt 48 includes a substrate made of a
ferromagnetic material (e.g., Ni), a thin-film elastic layer (e.g.,
silicone rubber) formed in the surface layer of the substrate, and
a mold-release layer (e.g., PFA) formed in the outer surface of the
elastic layer. The heating belt 48 may be a resin belt such as PI
if designed to have no heat-generation function. The heat roller 46
includes a metal core made of magnetic metal (e.g., Fe or SUS) and
a mold-release layer (e.g., PFA) formed in the surface of the metal
core.
[0056] The pressure roller 44 includes, for example, a metal core
made of Fe and Al, a Si rubber layer formed on the metal core, and
a fluororesin layer formed in the surface of the rubber layer. The
pressure roller 44 may be provided inside with, for example, a
halogen heater 44a.
[0057] The fixing unit 14 further includes an IH coil unit 50 (not
shown in FIG. 1) arranged outward from the heat roller 46 and the
heating belt 48. The IH coil unit 50 includes an induction heating
coil 52, a pair of arch cores 54, a pair of side cores 56 and a
center core 58.
[0058] [Coil]
[0059] In the example of FIG. 2, induction heating is conducted in
the arcuate part of the heating belt 48 wound around the heat
roller 46, and thereby, the induction heating coil 52 is arranged
on an imaginary arcuate surface along the arcuate part of the
heating belt 48. Further, the induction heating coil 52 extends
along the longitudinal direction of the heat roller 46 and covers
substantially the whole heat roller 46 in the longitudinal
directions of the heat roller 46. In practice, a resinous bobbin 53
extending in the longitudinal direction of the heat roller 46 is
arranged outward from the arcuate part of the heat roller 46, and
the induction heating coil 52 is arranged in a winding shape on the
bobbin 53. The bobbin 53 is molded into a semi-cylindrical shape
conforming to the peripheral surface of the heat roller 46, and the
material thereof may preferably be a heat-resistant resin (e.g.,
PPS, PET or LCP).
[0060] [Fixed Core]
[0061] As shown in FIG. 2, the center core 58 is in the middle, and
the pair of arch cores 54 and the pair of side cores 56 are on both
sides of the center core 58. The arch cores 54 are ferrite cores
(fixed core) which are symmetrically molded in an arch-shape in
section, and the full length thereof is greater than the length of
the winding region of the induction heating coil 52. The side cores
56 are ferrite cores (fixed core) molded in a block-shape, and each
side core 56 is connected to an end (lower end in FIG. 2) of the
corresponding arch core 54 and covers the outside (lower part in
FIG.2) of the winding region of the induction heating coil 52. The
arch core 54 is employed in a plural number. The arch cores 54 are
arranged apart from each other in a plurality of places in the
longitudinal direction of the heat roller 46 (refer to FIG. 3). The
side cores 56 are arranged along the longitudinal directions of the
heat roller 46 and have a full length corresponding to the length
of the winding region of the induction heating coil 52.
[0062] The arrangement of the cores 54 and 56 is determined, for
example, in accordance with the distribution of a magnetic-flux
density (magnetic-field strength) of the induction heating coil 52.
Although the arch cores 54 are arranged at predetermined intervals,
the side cores 56 compensate for a magnetic-focusing effect in
places where the arch cores 54 are not arranged, making the
magnetic-flux density distribution (temperature difference) in the
longitudinal direction of the heat roller 46 uniform. Outward from
the arch cores 54 and the side cores 56, for example, a resinous
core holder (not shown) is provided which supports the arch cores
54 and the side cores 56 and the material thereof may preferably be
a heat-resistant resin (e.g., PPS, PET or LCP).
[0063] The heat roller 46 is provided inside with a thermistor 62
which can be arranged especially in a place where the heat roller
46 generates a large quantity of heat by induction heating. The
thermistor 62 operates in response to an excessive temperature rise
in the heat roller 46 to stop the heating conducted by the
induction heating coil 52. Besides, a thermostat (not shown) can be
provided inside the heat roller 46, improving the safety at the
time of an abnormal temperature rise.
[0064] [Block-shaped Core]
[0065] The center core 58 is, for example, a ferrite core having a
cylindrical shape in section and a rotating-shaft member 59 is
inserted through the center of the center core 58 in the axial
direction of the center core 58. The rotating-shaft member 59 is
formed from, for example, a non-magnetic metal (AL or the like) or
a heat-resistant resin (PPS, PET, LCP or the like). The center core
58 is divided into a plurality of parts in the axial direction, and
each part is formed as a block-shaped core 58a (movable core).
[0066] As can be seen in FIG. 2, a driving roller 80 and a driving
motor 82 are provided above the center core 58 (on the opposite
side to the heat roller 46). The driving roller 80 is, for example,
formed on the surface with a rubber layer, and the outer peripheral
surface of the driving roller 80 is in contact with one
block-shaped core 58a. The rubber layer on the surface of the
driving roller 80 is pressed into contact with the surface of the
block-shaped core 58a with a moderate load by the elastic force of
a spring (not shown) or the like. The driving roller 80 is rotated
(driven) by the driving power of the driving motor 82, and this
rotation leads the block-shaped core 58a in contact therewith to
rotate with the friction force.
[0067] [Shielding Member]
[0068] The outer surface of each block-shaped core 58a is attached
with a shielding member 60. The shielding member 60 is a thin plate
member and is curved in an arcuate shape corresponding to the shape
of the outer surface of the center core 58a. The shielding member
60 may be, as shown in the figure, for example, embedded in the
block-shaped core 58a, or affixed to the outer surface of the
block-shaped core 58a. The shielding member 60 can be affixed, for
example, with a silicon adhesive.
[0069] It is preferable that the shielding member 60 is made of a
non-magnetic and electrically-conductive material, such as
oxygen-free copper. The shielding member 60 generates opposing
magnetic field by the influence of induction current induced when a
magnetic field perpendicular to a surface of the shielding member
60 penetrates the surface of the shielding member 60, and then
cancel interlinkage flux (perpendicular penetrating magnetic field)
to thereby shield the magnetic field. Further, by using a good
electrically conductive material, the generation of Joule heat by
the induction current is suppressed and the magnetic field can be
efficiently shielded. In order to improve electrical conductivity,
it is effective, for example, to select a material with as small a
specific resistance as possible and to increase the thickness of
the members. Specifically, it is preferable that the thickness of
the shielding member 60 is greater than 0.5 mm. The thickness of
the shielding member 60 is selected to be 1 mm in this
embodiment.
[0070] The center core 58 is arranged between the arch cores 54 and
the heat roller 46 (the heating belt 48) with respect to the
direction of the magnetic-field generation by the induction heating
coil 52 to form a magnetic path together with the arch cores 54 and
the side cores 56. In detail, an end 54a (magnetic-path inlet or
outlet) of the arch core 54 is apart from the heating belt 48, and
the center core 58 is a member forming an intermediate magnetic
path between the end 54a and the heating belt 48.
[0071] As shown in FIG. 2, if the shielding member 60 is in a
position (shielding position) adjacent to the surface of the
heating belt 48, the magnetic resistance rises around the induction
heating coil 52 to lower the magnetic-field strength. On the other
hand, if the block-shaped core 58a rotates (the direction is not
especially limited) by 180 degrees from the state of FIG. 2 and the
shielding member 60 is moved to a position (retracted position)
farthest away from the heating belt 48, the magnetic resistance
falls around the induction heating coil 52, leading formation of a
magnetic path through the arch cores 54 and the heat roller 46 on
both sides around the center core 58. As a result, the magnetic
field works on the heating belt 48 and the heat roller 46.
[0072] [Details of Center Core]
[0073] FIG. 3 is a plan view showing in detail a configuration of a
plurality of cores divided from the center core 58 in the axial
direction. The center core 58 extends in a width direction
orthogonal to the sheet conveying direction (shown by an arrow in
FIG. 3), in other words, in the width direction of a sheet and has
a full length slightly greater than a maximum conveyed-sheet width
(e.g., the longitudinal length of A3 or the lateral length of A4).
In this embodiment, the center core 58 includes four block-shaped
cores 58a, two block-shaped cores 58b at both ends in the width
direction, and one block-shaped core 58c in the middle in the width
direction. The block-shaped cores 58b at both ends are each
provided with the shielding member 60 while the block-shaped core
58c in the middle is not provided with the shielding member 60.
[0074] The block-shaped cores 58a, 58b and 58c are each arranged in
a predetermined position in the sheet-width direction. The
block-shaped core 58c may be divided into several core pieces in
the axial direction if it is too large, thereby facilitating the
manufacturing thereof.
[0075] [Cores at Both Ends and in the Middle]
[0076] The rotating-shaft member 59 penetrates the whole center
core 58 and extends in the axial direction of the center core 58,
and has a full length greater than the center core 58. Among the
block-shaped cores 58a, 58b and 58c, two block-shaped cores 58b at
both ends and the middle block-shaped core 58c in the width
direction are fixed to the rotating-shaft member 59, and thereby,
three block-shaped cores 58b and 58c are rotated together as the
rotating-shaft member 59 rotates.
[0077] The IH coil unit 50 is provided with a driving motor 66
whose driving power rotates the rotating-shaft member 59. A driven
gear 59a is attached to an end of the rotating-shaft member 59 and
engaged with an output gear 66a of the driving motor 66. As the
driving motor 66 is driven, the driving power rotates the
rotating-shaft member 59, thereby rotating the three block-shaped
cores 58b and 58c together.
[0078] [Independent Cores]
[0079] The four block-shaped cores 58a are all penetrated in the
axial direction by the rotating-shaft member 59 and supported so as
to be loosely rotated relative to the rotating-shaft member 59.
Therefore, the driving motor 82 provided in each block-shaped core
58a is driven to rotate each block-shaped core 58a individually and
independently.
[0080] FIGS. 4A and 4B show an operation as the center core 58
(particularly, the block-shaped cores 58a and 58b) rotates, and
each of them will be below described.
[0081] FIG. 4A shows an operation in the case where the shielding
member 60 is switched to the retracted position as each
block-shaped core 58a, 58b rotates. In this case, a magnetic field
generated by the induction heating coil 52 passes the heating belt
48 and the heat roller 46 through the side cores 56, the arch cores
54 and each block-shaped core 58a, 58b. At this time, the
ferromagnetic heating belt 48 and heat roller 46 cause an eddy
current and generate Joule heat based on the specific resistance of
each material, conducting heating.
[0082] FIG. 4B shows an operation in the case where the shielding
member 60 is switched to the shielding position. In this case, the
shielding member 60 of each block-shaped core 58a, 58b is located
on a magnetic path in the width direction of the center core 58,
thereby partly suppressing generation of a magnetic field. This
suppresses the quantity of heat generated in the position of each
block-shaped core 58a, 58b, preventing an excessive temperature
rise in the heating belt 48 or the heat roller 46.
[0083] [Rotation Control Method]
[0084] Next, a description will be given about a method of
individually controlling the rotation of each block-shaped core
58a, 58b, 58c of the center core 58. FIGS. 5A and 5B are a side
view showing one end of the center core 58 and a partial sectional
view (longitudinal section along a B-B line in FIG.5B) showing an
operation thereof, respectively.
[0085] As shown in FIG. 5A, the rotating-shaft member 59 is
provided at the other end with a position detecting member 73
protruding in a radial direction from the outer surface thereof, as
well as two photo-interrupters 74 on both sides of the other end.
In this embodiment, the stop position in which the driving motor 66
stops is controlled based upon a detection signal from the
photo-interrupters 74 to rotate the block-shaped cores 58b and 58c
at both ends and in the middle of the center core 58 by 180 degrees
and switch the positions of the shielding members 60. The
block-shaped core 58c in the middle is not provided with the
shielding member 60, and in the block-shaped cores 58b at both
ends, the positions of the shielding members 60 are switched. Each
block-shaped core 58a is provided on the outer-circumference
surface with a position detecting member 84 protruding in a radial
direction from the outer peripheral surface.
[0086] As shown in FIG. 5B, the block-shaped core 58a is provided
on both sides with photo-interrupters 86. On the basis of a
detection signal from the photo-interrupters 86, the stop position
in which each corresponding driving motor 82 stops is controlled to
rotate each block-shaped core 58a individually by 180 degrees and
switch the position (shielding position or retracted position) of
the shielding member 60 separately.
[0087] The shielding position and the retracted position are
mutually opposite positions 180 degrees apart from each other, and
the shielding member 60 is moved to the shielding position or the
retracted position by switching the rotation direction of the
driving roller 80 in a forward and reverse manner. For example, as
shown in FIG. 5B, if the block-shaped core 58a is rotated clockwise
to move the shielding member 60 to the shielding position, the
position detecting member 84 is detected by one photo-interrupter
86 and simultaneously the block-shaped core 58a is prevented from
overrunning by the position detecting member 84. On the other hand,
if the block-shaped core 58a is rotated counterclockwise to move
the shielding member 60 to the retracted position, the position
detecting member 84 is detected by the other photo-interrupter 86
and the block-shaped core 58a is prevented from overrunning by the
position detecting member 84.
[0088] Each block-shaped core 58a, 58b is provided with two
photo-interrupters 86, 74. Instead of this constitution, however,
the shielding position of each block-shaped core 58a, 58b is set as
a reference position and one photo-interrupter 86, 74 may be
arranged in a position where the position detecting member 73, 84
is detected. In this case, the position in which each driving motor
82, 66 stops is controlled in such a way that the position where
each block-shaped core 58a, 58b is rotated by 180 degrees from the
reference position (shielding position) becomes the retracted
position.
[0089] [Individual Control Circuit]
[0090] In this embodiment, each driving motor 66, 82 may be, for
example, a stepping motor and the operation thereof is controlled
by a control circuit (not shown). This control circuit can be
formed, for example, by a control IC, an I/O driver, a
semiconductor memory and the like. A detection signal from each
photo-interrupter 74, 86 is inputted via the input driver to the
control IC, and on the basis of detection signal, the control IC
detects a present rotation angle (position) of each driving motor
66, 82. The control IC is notified of information on a present
sheet size from an image forming control unit (not shown). Upon
receiving the information, the control IC reads information on the
position (shielding position or retracted position) of the
shielding member 60 suitable for the sheet size from the
semiconductor memory (ROM) and outputs a drive pulse equivalent to
the rotation angle (180 degrees) corresponding to the position
information at that time. The drive pulse is applied to each
driving motor 66, 82 via the output driver to operate each driving
motor 66, 82.
[0091] [Individual Control Example]
[0092] Next, a description will be given about the control of each
block-shaped core 58a, 58b, 58c in accordance with the size of a
sheet. In this embodiment, each block-shaped core 58a, 58b, 58c is
designed in size to correspond to each conveyed-sheet width
equivalent to, for example, the longitudinal length of A5, A4 or
B4, or the lateral length of A4.
[0093] FIGS. 6A and 6B show a control example in accordance with
each of a minimum conveyed-sheet width and a maximum conveyed-sheet
width. The conveyed-sheet width denotes a sheet conveyed region
where a sheet passes in accordance with the size of the sheet,
particularly, the width orthogonal to a sheet-conveying direction.
The outer surface of each block-shaped core 58a, 58b, 58c is given
halftone dots. Each control example will be below described.
[0094] [Minimum Conveyed-Sheet Width]
[0095] As shown in FIG. 6A, when image formation is conducted with
a minimum conveyed-sheet width W1 (e.g., the longitudinal length of
A5), the stop positions (rotation angles) of the driving motors 66
and 82 are controlled with each shielding member 60 of the
block-shaped cores 58b at both ends and the four block-shaped cores
58a switched to the shielding position. In this case, although the
heat roller 46 is induction heated within the range of the minimum
conveyed-sheet width W1, heat generation is suppressed outside the
minimum conveyed-sheet width W1, preventing an excessive
temperature rise in the heat roller 46.
[0096] [Maximum Conveyed-Sheet Width]
[0097] As shown in FIG. 6B, when image formation is conducted with
a maximum conveyed-sheet width W4 (e.g., the lateral length of A4
or the longitudinal length of A3), the stop positions (rotation
angles) of the driving motors 66 and 82 are controlled with each
shielding member 60 of the block-shaped cores 58b at both ends and
the four block-shaped cores 58a switched to the retracted position.
In this case, the heat roller 46 is induction heated within the
full range of the maximum conveyed-sheet width W4, and thereby, an
image can be securely fixed on a sheet having a maximum size.
[0098] [Intermediate Conveyed-Sheet Width]
[0099] FIGS. 7A to 7G show a control example in accordance with an
intermediate conveyed-sheet width. FIGS. 7B to 7G are each a
sectional view along a B-B line to a G-G line of FIG. 7A. The
following description shows an operation from the state shown in
FIG. 6B.
[0100] As shown in FIG. 7A, when image formation is conducted with
an intermediate conveyed-sheet width W2 (e.g., the longitudinal
length of A4) one-size greater than the minimum conveyed-sheet
width W1, the stop positions (rotation angles) of the driving
motors 66 and 82 are controlled with each shielding member 60 of
the block-shaped cores 58b at both ends and the two block-shaped
cores 58a adjacent to the block-shaped cores 58b switched to the
shielding position. Specifically, each will be below described.
[0101] [Cores at Both Ends]
[0102] As shown in FIGS. 7B and 7G, the block-shaped cores 58b are
rotated by 180 degrees together with the rotating-shaft member 59
by the drive of the driving motor 66 to thereby switch each
shielding member 60 of the block-shaped cores 58b to the shielding
position.
[0103] [Two Cores Near Both Ends]
[0104] As shown in FIGS. 7C and 7F, the block-shaped cores 58a
adjacent to the block-shaped cores 58b are rotated by 180 degrees
individually by the corresponding driving motor 82 to thereby
switch the shielding members 60 to the shielding positions.
[0105] [Two Cores near Middle]
[0106] As shown in FIGS. 7D and 7E, the two block-shaped cores 58a
adjacent to the block-shaped core 58c in the middle of the center
core 58 are kept in the retracted positions.
[0107] Next, FIGS. 8A to 8G show another control example in
accordance with an intermediate conveyed-sheet width. FIGS. 8B to
8G are each a sectional view along a B-B line to a G-G line of FIG.
8A. The following description shows an operation from the state
shown in FIG. 7.
[0108] As shown in FIG. 8A, when image formation is conducted with
an intermediate conveyed-sheet width W3 (e.g., the longitudinal
length of B4) one-size greater than the intermediate conveyed-sheet
width W2 and one-size smaller than the maximum conveyed-sheet width
W4, the stop position (rotation angle) of the driving motor 66 is
controlled in such a way that each shielding member 60 of the
block-shaped cores 58b at both ends is switched to the shielding
position, and the stop position (rotation angle) of each driving
motor 82 is controlled in such a way that each shielding member 60
of the four block-shaped cores 58a is switched to the retracted
position. Specifically, each will be below described.
[0109] [Cores at Both Ends]
[0110] As shown in FIGS. 8B and 8G, the shielding members 60 of the
block-shaped cores 58b at both ends of the center core 58 are kept
in the shielding positions.
[0111] [Two Cores Near Both Ends]
[0112] As shown in FIGS. 8C and 8F, the block-shaped cores 58a
adjacent to the two block-shaped cores 58b are rotated by 180
degrees individually by the corresponding driving motors 82 to
thereby switch the shielding member 60 to the retracted
position.
[0113] [Two Cores Near Middle]
[0114] As shown in FIGS. 8D and 8E, the two block-shaped cores 58a
adjacent to the block-shaped core 58c in the middle of the center
core 58 are kept in the retracted positions.
[0115] [Magnetism Adjustment Unit]
[0116] In this embodiment, the rotating-shaft member 59 supporting
the block-shaped cores 58b and 58c, the driving motor 66 driving
the rotating-shaft member 59, the driving roller 80 pressed into
contact with each peripheral surface of the block-shaped cores 58a
and the driving motor 82 driving the driving roller 80 constitute a
magnetism adjustment unit capable of switching each shielding
member 60 of the cores 58a and 58b between the shielding position
and the retracted position. The magnetism adjustment unit
individually rotates the four block-shaped cores 58a and
independently controls the position (shielding position and
retracted position) of each shielding member 60, thereby adjusting
the quantity of screened-out magnetism optimally in accordance with
the intermediate conveyed-sheet widths W2 and W3 of various types.
This makes it possible to control the heated range of the heat
roller 46 precisely in accordance with the size (conveyed-sheet
width) of the sheet determined in advance and to prevent an
excessive temperature rise certainly outside the conveyed-sheet
width. In some of the above figures, although clockwise and
counterclockwise rotations are each shown by an arrow, each
block-shaped core 58a, 58b may be rotated only in one direction,
and further, the sheet-conveying direction may be opposite to the
direction shown in some of the figures.
[0117] [Other Structural Examples]
[0118] FIG. 9 shows a further structural example of the fixing unit
14 which fixes a toner image using the fixing roller 45 and the
pressure roller 44 without any heating belt. For example, a
magnetic body similar to the above heating belt is wound around the
outer periphery of the fixing roller 45 and subjected to induction
heating by the induction heating coil 52. In this case, the
thermistor 62 is arranged outside the fixing roller 45 so as to
face the magnetic-body layer. This structural example has the same
as the above and is capable of managing changes in the size of a
sheet by rotating each block-shaped core 58a, 58b.
[0119] Next, FIG. 10 shows a further structural example of the IH
coil unit 50 which conducts induction heating, not in the arcuate
shape part of the heating belt 48, but in a flat part of the
heating belt 48 between the heat roller 46 and the fixing roller
45. This structural example is also capable of managing changes in
the size of a sheet by rotating each block-shaped core 58a,
58b.
[0120] Diverse variations are feasible in this embodiment. Each
block-shaped core 58a, 58b, 58c has a cylindrical or columnar shape
but is not limited to this, and hence, may have a polygonal shape
in section. Further, the length of each block-shaped core 58a, 58b,
58c in the axial directions is not especially restricted, and
hence, may be set suitably for the size of a sheet in use.
[0121] Besides, the specific form of each component element
including the arch core 54 or the side core 56 is not limited to
the one shown in the figures, and hence, may be properly
variable.
Second Embodiment
[0122] [Details of Fixing Unit]
[0123] Next, the fixing unit 14 of the image forming apparatus 1
according to a second embodiment of the present invention will be
described in detail.
[0124] FIG. 11 is a longitudinal sectional view showing the fixing
unit 14 according to the second embodiment. In the same way as the
first embodiment, the fixing unit 14 according to the second
embodiment includes, as basic component elements thereof, the
pressure roller 44, the fixing roller 45, the heat roller 46 and
the heating belt 48. Hence, those members 44, 45, 46 and 48 are not
described here.
[0125] The fixing unit 14 further includes an IH coil unit 150
outside the heat roller 46 and the heating belt 48. The IH coil
unit 150 includes an induction heating coil 52, a pair of arch
cores 54, a pair of side cores 56 and a center core 158. The
induction heating coil 52, arch cores 54 and side cores 56 of the
IH coil unit 150 have configurations substantially similar to the
IH coil unit 50 according to the first embodiment, and hence, the
description thereof is omitted. The center core 158 will be below
described in detail.
[0126] [Center Core]
[0127] The center core 158 is, for example, a ferrite core having a
cylindrical shape in section and includes a shaft member 159
inserted through the center thereof in the axial direction. The
shaft member 159 is formed from, for example, a non-magnetic metal
(AL or the like) or a heat-resistant resin (PPS, PET, LCP or the
like). The center core 158 is divided into a plurality of parts to
form a plurality of block-shaped cores 158a. The cores 158a are
arranged in the axial direction of the center core 158.
[0128] [Shielding Member]
[0129] Each block-shaped core 158a has a shielding member 60
attached to the outer surface thereof. The shielding member 160 is
a thin plate member and is curved in an arcuate shape conforming to
the shape of the outer surface of the core 158a. The shielding
member 160 maybe, as shown in the figure, for example, embedded in
the block-shaped core 158a, or affixed to the outer surface of the
block-shaped core 158a. The shielding member 60 can be affixed, for
example, with a silicon adhesive.
[0130] It is preferable that the shielding member 160 is made of a
non-magnetic and electrically-conductive material, such as
oxygen-free copper. In the shielding member 160, a magnetic field
perpendicular to the surface thereof penetrates to cause an induced
current and thereby generate a reverse magnetic field and cancel an
interlacing magnetic flux (perpendicular penetration magnetic
field), thereby screening out a magnetic field. Further, an
electrically-conductive member is employed, thereby suppressing
Joule heat generation caused by an induced current to screen out
the magnetic field efficiently. In order to improve the electrical
conductivity, for example, it is effective to select a material
having a low specific resistance and thicken the member, and
specifically, the thickness of the shielding member 160 may
preferably be 0.5 mm or above, and for example, it is 1 mm in the
second embodiment.
[0131] As shown in FIG. 11, if the shielding member 160 is in a
position (shielding position) adjacent to the surface of the
heating belt 48, the magnetic resistance rises around the induction
heating coil 52 to lower the magnetic-field strength. On the other
hand, if the block-shaped core 158a rotates (the direction is not
especially limited) by 180 degrees and by the shielding member 160
is moved to a position (retracted position) farthest away from the
heating belt 48, the magnetic resistance falls around the induction
heating coil 52, leading formation of a magnetic path passing
through the center core 158, the arch cores 54 and the heat roller
46 on both sides of the center core 158. As a result, the magnetic
field works on the heating belt 48 and the heat roller 46.
[0132] [Details of Center Core]
[0133] FIGS. 12A to 12E are plan views showing in detail a
configuration of the center core 158 divided in the axial
direction. FIG. 12A and FIGS. 12C to 12E show a state in which the
shaft member 159 is separated (extracted) from the center core 158.
As is not shown in the figures, the center core 158 extends in the
width direction of a sheet and has a full length (reference
character L in FIG. 12A) greater than a maximum conveyed-sheet
width (e.g., the longitudinal length of A3 or the lateral length of
A4). If the longitudinal direction of the center core 158 shown in
FIG. 12A is regarded as the axial direction (not shown), the axial
direction corresponds to the sheet-width direction.
[0134] Although the block-shaped cores 158a arranged in the middle
of the center core 158 in the axial direction are omitted in FIG.
12A, the center core 158 is divided into, for example, ten parts,
in other words, ten block-shaped cores 158a form the center core
158. In FIG. 12A, all the block-shaped cores 158a are provided with
the shielding members 160, but the middle block-shaped cores 158a
arranged within a minimum conveyed-sheet width (W1 in the figures)
may not be provided with the shielding member 160.
[0135] [Axial Groove]
[0136] As shown in FIGS. 12A and 12B, each block-shaped core 158a
is formed with a through path 158b penetrating the inside thereof
in the axial direction and having a circle-shape in section in the
axial direction. The shaft member 159 is inserted through the
through path 158b in the axial direction. Further, the
inner-circumference surface of each block-shaped core 158a is
formed with an axial groove 158c extending along the through path
158b in the axial direction. The axial groove 158c has a
quadrilateral-shape in section when seen in the axial direction of
the center core 158.
[0137] [Circumferential Groove]
[0138] Among the ten block-shaped cores 158a, some of the cores
158a has an inner peripheral surface formed with a circumferential
groove 158d extending in the circumferential directions of each
core 158a. In FIG. 12A, the circumferential groove 158d is formed
in the second and third block-shaped cores 158a from both ends of
the center core 158 in the axial direction of the center core 158.
Specifically, the second block-shaped cores 158a from both ends are
each formed with one circumferential groove 158d while the third
block-shaped cores 158a from both ends are each formed with two
circumferential grooves 158d. The circumferential groove 158d has a
quadrilateral-shape in section when seen in the axial direction and
extends over a predetermined angle (e.g., approximately 180
degrees) in the circumferential direction of the block-shaped core
158a from the axial groove 158c.
[0139] The shaft member 159 is shaped like a round bar and has a
full length greater than that of the center core 158. The outer
diameter of the shaft member 159 is slightly smaller than the inner
diameter of the block-shaped core 158a, in other words, the
diameter of the through path 158b, thereby enabling each
block-shaped core 158a to rotate along the outer peripheral surface
of the shaft member 159. The shaft member 159 can move or slide
relative to the center core 158 (block-shaped core 158a) in the
axial direction. The shaft member 159 is capable of moving in the
axial direction by a moving mechanism 180 (FIG. 14) and rotating
around the axial center by a rotation mechanism 164 (FIG. 14).
[0140] [Projection]
[0141] The shaft member 159 is provided on the outer peripheral
surface with a plurality of projections 159a, 159b and 159c that
are arranged at a predetermined interval and on the same line in
the axial direction of the shaft member 159. The projections 159a,
159b and 159c have substantially the same shape and size.
[0142] The shape and size of each projection 159a, 159b, 159c are
set in such a way that they can be received in the axial groove
158c and the circumferential groove 158d. Therefore, as shown in
FIG. 12B, with the shaft member 159 inserted through the through
paths 158b, all the projections 159a, 159b and 159c are received in
the axial grooves 158c, thereby allowing the shaft member 159 to
move in the axial direction relative to the block-shaped cores 158a
inside of the through paths 158b.
[0143] If the shaft member 159 is rotated relative to the
block-shaped cores 158a with any of the projections 159a, 159b and
159c aligned in the axial direction with the circumferential groove
158d, the projections 159a, 159b and 159c are received into the
circumferential groove 158d and moved in the circumferential
direction along the circumferential groove 158d.
[0144] [Magnetism Adjustment Method]
[0145] Each shielding member 160 of the block-shaped cores 158a is
switched from the retracted position to the shielding position in
accordance with the size of a sheet to be printed. In FIGS. 12A and
12B, the shielding members 160 are in the retracted positions, and
if each block-shaped core 158a is rotated by 180 degrees around the
axial center, the shielding members 160 are moved from the
retracted positions to the shielding positions shown in FIG.
11.
[0146] When the block-shaped core 158a is required to have the
shielding member 160 switched to the shielding position, the shaft
member 159 is rotated with the projection 159a, 159b or 159c
received in the axial groove 158c to thereby rotate the
block-shaped core 158a together with the shaft member 159. On the
other hand, when the block-shaped core 158a is not required to have
the shielding member 160 switched to the shielding position, the
shaft member 159 is rotated with the projection 159a, 159b or 159c
received in the circumferential groove 158d. In this case, since
the projection 159a, 159b or 159c moves along the circumferential
groove 158d, the block-shaped core 158a is not rotated (or idled)
even if the shaft member 159 is rotated. A description will be
below given about a switch from the retracted position to the
shielding position in accordance with the size of a sheet.
[0147] [Minimum Sheet-Conveyed Region W1]
[0148] As shown in FIG. 12C, when the size of a sheet is minimum
(e.g., the longitudinal length of A5), all the six block-shaped
cores 158a on both outsides of a minimum sheet conveyed region W1
are rotated, executing control in such a way that the shielding
members 160 switch from the retracted positions to the shielding
positions. Specifically, the shaft member 159 is moved in the axial
direction to bring the projections 159a, 159b and 159c to positions
where they do not align with the circumferential grooves 158d. If
the shaft member 159 is rotated in this state, each projection
159a, 159b, 159c is rotated while being hooked in the axial groove
158c, thereby rotating all the six block-shaped cores 158a outside
the minimum sheet conveyed region W1.
[0149] [Intermediate Sheet Conveyed Region W2]
[0150] As shown in FIG. 12D, when the size of a sheet is
intermediate (e.g., the longitudinal length of A4), the four
block-shaped cores 158a on both outsides of an intermediate sheet
conveyed region W2 are rotated, executing control in such a way
that the shielding members 160 switch from the retracted positions
to the shielding positions. Specifically, the shaft member 159 is
moved in a predetermined direction (rightward in FIG. 12C) from the
position shown in FIG. 12C to bring the circumferential grooves
158d of the two block-shaped cores 158a inside the intermediate
sheet conveyed region W2 and the projections 159a on the same lines
(L1 in the figure) perpendicular to the axial center. If the shaft
member 159 is rotated in this state, the four projections 159b and
159c are rotated while being hooked in the axial grooves 158c
whereas the two projections 159a are moved along the
circumferential grooves 158d without being hooked in the axial
grooves 158c. As a result, only the four block-shaped cores 158a
outside the intermediate sheet conveyed region W2 are rotated while
the other block-shaped cores 158a inside the intermediate
sheet-conveyed region w2 are not rotated.
[0151] [Maximum Sheet Conveyed Region W3]
[0152] As shown in FIG. 12E, when the size of a sheet is maximum
(e.g., the lateral length of A4), only the two block-shaped cores
158a on both outsides (at both ends of the center core 158) of a
maximum sheet conveyed region W3 are rotated, executing control in
such a way that the shielding members 160 switch from the retracted
positions to the shielding positions. Specifically, the shaft
member 159 is further moved rightward from the position shown in
FIG. 12D to bring the circumferential grooves 158d of the two
block-shaped cores 158a (the third cores 158a from both ends of the
center core 158) near the middle inside of the maximum sheet
conveyed region W3 and the projections 159a on the same lines (L2
in the figure) and also to bring the circumferential grooves 158d
of the two block-shaped cores 158a (the second center cores 158a
from both ends of the core 158) and the projections 159a on the
same lines (L3 in the figure). If the shaft member 159 is rotated
in this state, the two projections 159c are each hooked in the
axial groove 158c of the outermost core 158a while the four
projections 159a and 159b are moved along the circumferential
grooves 158d without being hooked in the axial grooves 158c. As a
result, only the two block-shaped cores 158a outside the maximum
sheet conveyed region W3 are rotated while the other block-shaped
cores 158a are not rotated. FIGS. 13A to 13D are vertical sectional
views showing a rotation or non-rotation state of the block-shaped
core 158a as the shaft member 159 rotates. FIGS. 13A and 13B are
sectional views along an A-A line of FIG. 12, and FIGS. 13C and 13D
are sectional views along a B-B line of FIG. 12.
[0153] [Switch to Shielding Position]
[0154] As shown in FIG. 13A, the shaft member 159 can move in the
axial direction with the projection 159c received in the axial
groove 158c, and with respect to the rotation direction, the
projection 159c is hooked in the axial groove 158c.
[0155] As shown in FIG. 13B, if the shaft member 159 is, for
example, rotated clockwise by 180 degrees, the block-shaped core
158a is rotated together with the shaft member 159 by the
projection 159c received in the axial groove 158c, thereby
switching the shielding member 160 from the retracted position to
the shielding position. The shielding member 160 can be returned
from the shielding position to the retracted position by rotating
the shaft member 159 reversely by 180 degrees.
[0156] [Keeping in Retracted Position]
[0157] As shown in FIG. 13C, if the projections 159a and 159b are
aligned with the circumferential grooves 158d, the projections 159a
and 159b are unhooked from the cores 158a.
[0158] As shown in FIG. 13D, if the shaft member 159 is, for
example, rotated clockwise by 180 degrees, the projections 159a and
159b only move in the circumferential direction in the
circumferential grooves 158d and the block-shaped cores 158a are
not rotated together, thereby keeping the shielding member 160 in
the retracted position. In this state, even if the shaft member 159
is reversely rotated by 180 degrees, each projection 159a, 159b
moves reversely in the circumferential groove 158d and merely
returns into the axial groove 158c to keep the block-shaped core
158a unturned.
[0159] [Rotation Mechanism, Moving Mechanism]
[0160] Next, a configuration will be described for rotating or
moving the shaft member 159. FIGS. 14 and 15 are each a side view
showing a configuration of the rotation mechanism 164 and the
moving mechanism 180 of the shaft member 159, and in FIG. 14, the
center core 158 is shown in a longitudinal section.
[0161] The rotation mechanism 164 rotates the shaft member 159, for
example, by transmitting the rotation of a stepping motor 166 via
gears 167 and 168 to drive a drive shaft 170. In order to detect a
rotation position (reference position in the rotation direction) of
the shaft member 159, the gear 168 is provided on a side thereof
with an index 172 and a photo-interrupter 174 combined
therewith.
[0162] The drive shaft 170 is integrally connected with the shaft
member 159 and has the same axial center as those of the shaft
member 159 and the center core 158. The rotation angle (switch
between the retracted position and the shielding position) of the
shaft member 159 can be controlled, for example, with a drive-pulse
number applied to the stepping motor 166, and the rotation
mechanism 164 has a control circuit (not shown) for this purpose.
The control circuit can be formed, for example, by a control IC, an
I/O driver, a semiconductor memory and the like. A detection signal
from the photo-interrupter 174 is inputted via the input driver to
the control IC, and on the basis of the detection signal, the
control IC can detect the shaft member 159 being in the reference
position or not. In the second embodiment, the shielding member 160
stops in the retracted position as the shaft member 159 stops in
the reference position, and the shielding member 160 is switched
from the retracted position to the shielding position as the shaft
member 159 is rotated by 180 degrees from the reference
position.
[0163] The moving mechanism 180 moves the shaft member 159 in the
axial direction through the drive shaft 170, for example, by
transmitting the mechanical power of a stepping motor 182 via gears
184 and 185 to rotate a swash plate cam 186 which in turn drives
the drive shaft 170. The swash plate cam 186 is formed with a cam
plane 186a inclined with respect to an axial line thereof, and an
end of the drive shaft 170 is in contact with the cam plane 186a to
form a sliding pair therewith. The drive shaft 170 has a
compression coil spring 188 connected to the other end (near the
rotation mechanism 164) thereof and is given an initial thrust (or
biasing force) by a repulsive force of the spring 188. Hence, the
swash plate cam 186 is rotated to reciprocate the drive shaft 170
in the axial direction, thereby allowing the shaft member 159 to go
and return in the axial direction. Although the other end of the
drive shaft 170 penetrates the gear 168 of the rotation mechanism
164, the gear 168 and the drive shaft 170 are subjected to spline
coupling using a key 171, thereby hindering the gear 168 from
moving in the axial direction even if the drive shaft 170 moves in
the axial direction.
[0164] The center core 158 is provided at both ends with sleeves
163 restricting the movement thereof in the axial direction. On the
other hand, the shaft member 159 is provided at both ends with
collar members 161 each fitted along the inner peripheral surface
of the corresponding sleeve 163. When the shaft member 159 is moved
in the axial direction, the collar members 161 are guided by the
sleeves 163, realizing a smooth movement thereof.
[0165] [Control Method]
[0166] The stop position (movement distance) of the shaft member
159 in the axial direction varies according to the rotation angle
of the swash plate cam 186. The stop position of the shaft member
159 can be controlled, for example, with a drive-pulse number
applied to the stepping motor 182. The moving mechanism 180 also
has a control circuit (not shown). This control circuit can also be
formed, for example, by a control IC, an I/O driver, a
semiconductor memory and the like and has control information on
stop positions of the shaft member 159 according to sheet sizes
stored in advance in the semiconductor memory (e.g., EEPROM). The
control IC is notified of information on a present sheet size from
an image forming control unit (not shown). Upon receiving the
information, the control IC reads from the semiconductor memory
information on the stop position of the shaft member 159 suitable
for the sheet size and outputs, at a specified cycle, the
predetermined number of drive pulses for allowing the shaft member
159 to reach the targeted stop position. The drive pulse is applied
to the stepping motor 182 via the output driver to operate the
stepping motor 182.
[0167] After confirming that the rotation mechanism 180 has
finished controlling the stop position of the shaft member 159, the
control circuit of the rotation mechanism 164 rotates the stepping
motor 166. As described earlier, the block-shaped cores 158a
outside the sheet conveyed region are rotated according to the
sheet size at that time to switch the shielding members 160 from
the retracted positions to the shielding positions.
[0168] FIGS. 16A and 16B show an operation of the block-shaped core
158a as the shaft member 159 rotates. FIG. 16A shows the shielding
member 160 held in the retracted position, and in this case, a
magnetic field generated by the induction heating coil 52 passes
the heating belt 48 and the heat roller 46 through the side cores
56, the arch cores 54 and the center core 158. At this time, the
ferromagnetic heating belt 48 and heat roller 46 cause an eddy
current and generate Joule heat based on the specific resistance of
each material, thereby conducting heating.
[0169] FIG. 16B shows the shielding member 160 switched to the
shielding position, and in this case, the shielding member 160 is
located on a magnetic path outside the sheet conveyed region
according to the sheet size, thereby suppressing generation of a
magnetic field. This suppresses the quantity of heat generated
outside the sheet conveyed region, thereby preventing an excessive
temperature rise in the heating belt 48 and the heat roller 46.
Third Embodiment
[0170] FIG. 17 shows the fixing unit 14 according to a third
embodiment of the present invention in which the shielding member
160 is replaced with a cut-out portion 90 formed in each of the
block-shaped core 158a, and hence, each block-shaped core 158a has
an arcuate shape in section.
[0171] [Cut-Out Portion]
[0172] The cut-out portion 90 is formed by cutting off a part of
the block-shaped core 158a along the axial direction of the core
158a. The cut-out portion 90 may be formed in a molding die
simultaneously when sintering ferrite powder, or formed by cutting
a molded column (cylinder). As long as the cut-out portion 90 has
an arcuate shape in section in the final form, the manufacturing
process is not limited.
[0173] In the third embodiment, the block-shaped core 158a is
formed inside with the axial groove 158c and the circumferential
groove 158d, and the shaft member 159 is formed with the
projections 159a, 159b and 159c. The axial groove 158c and the
circumferential groove 158d are located, however, out of the way of
the cut-out portion 90. The rotation or non-rotation of the
block-shaped core 158 a by the projections 159a, 159b and 159c is
the same as that in the second embodiment.
[0174] In the third embodiment, control is executed in such a way
that the block-shaped cores 158a outside the sheet conveyed region
in accordance with the size of a sheet are rotated to switch the
cut-out portion 90 from the retracted position to a resistance
position (shielding position). Specifically, as shown in FIG. 17,
the cut-out portion 90 moves to a position (resistance position)
adjacent to the surface of the heating belt 48 to increase the
magnetic resistance around the induction heating coil 52 and lower
the magnetic field strength. Therefore, in the same way as the case
where the shielding member 160 is switched to the shielding
position in the second embodiment, an excessive temperature rise in
the heating belt 48 and the heat roller 46 can be certainly
prevented outside the sheet conveyed region.
[0175] On the other hand, when the cut-out portion 90 is in the
position 180 degrees apart from the position of FIG. 17 or in the
position (retracted position) farthest away from the heating belt
48, the magnetic resistance falls around the induction heating coil
52, leading formation of a magnetic path passing through the center
core 158, the arch cores 54 and the heat roller 46 on both sides of
the center core 158. As a result, the magnetic field works on the
heating belt 48 and the heat roller 46. In this case, in the same
way as the second embodiment, heat generation necessary for fixing
an image can be obtained.
[0176] FIG. 18 shows a further structural example of the fixing
unit 14 which fixes a toner image between the fixing roller 45 and
the pressure roller 44 without using any heating belt. For example,
a magnetic body similar to the above heating belt is wound around
the outer periphery of the fixing roller 45 and subjected to
induction heating by the induction heating coil 52. In this case,
the thermistor 62 is arranged outside the fixing roller 45 so as to
face the magnetic-body layer. This structural example has the same
as the second embodiment and is capable of screening out magnetism
outside the sheet conveyed region by rotating the block-shaped core
158a together with the shaft member 159.
[0177] FIG. 19 shows a still further structural example of the
fixing unit 14 which is different from the second embodiment in
that the heat roller 46 is made of a non-magnetic metal (e.g., SUS:
stainless steel) and the center core 158 is arranged inside the
heat roller 46. Further, the arch core 54 is employed in an
integral form and an intermediate core 55 is provided between the
arch core 54 and the heating belt 48.
[0178] Since the heat roller 46 is a non-magnetic metal, a magnetic
field generated by the induction heating coil 52 passes through the
side cores 56, the arch core 54 and the intermediate core 55,
penetrates the heat roller 46 and reaches the center core 158
inside of the heat roller 46. The penetration magnetic field gives
induction heating to the heating belt 48.
[0179] In this structural example, as shown in FIG. 19, the
shielding member 160 is switched to the position (shielding
position) facing the intermediate core 55 to screen out magnetism,
thereby suppressing an excessive temperature rise outside the sheet
conveyed region. On the other hand, if the shielding member 160 is
moved to the opposite side farthest away from the intermediate core
55 and comes to the retracted position, the heating belt 48
undergoes induction heating.
[0180] Next, FIG. 20 shows a further structural example of the IH
coil unit 150 which conducts induction heating, not in the arcuate
surface of the heating belt 48, but in a flat surface between the
heat roller 46 and the fixing roller 45. In the same way as the
second embodiment, this structural example is capable of screening
out magnetism outside the sheet conveyed region by rotating the
block-shaped cores 158a together with the shaft member 159.
[0181] FIG. 21 shows a structural example of an internal type IH
coil unit. In all the above examples, the induction heating coil 52
is arranged so as to surround the heat roller 46 while in an
internal type IH coil unit 250, the whole induction heating coil 52
is arranged inside of the heat roller 46.
[0182] The internal type IH coil unit 250 includes only the center
core 158 without such an arch core nor a side core as described
above. A magnetic field generated by the induction heating coil 52
passes the peripheral surface of the heat roller 46 and enters the
center core 158, then passes through the middle of the induction
heating coil 52 from the center core 158, and reaches a vicinity of
the nip between the heat roller 46 and the pressure roller 44.
Although the center core 158 is inside of the heat roller 46, in
the same way as the second embodiment, the block-shaped cores 158a
are rotated together with the shaft member 159, thereby screening
out magnetism outside the sheet conveyed region.
[0183] Diverse variations are feasible in the second embodiment and
the third embodiments. For example, the number of the block-shaped
cores 158a obtained by division is not limited especially to the
embodiments, and hence, may be varied suitably for the size of a
sheet in use.
[0184] In the first to third embodiments, the plate-shaped
shielding member 160 is employed to adjust (screen out) magnetism.
However, the shielding member 160 may be made of a non-magnetic
metal (e.g., oxygen-free copper) and have a closed-ring shape. In
this case, in the shielding member 160, a magnetic flux penetrating
the closed ring generates a magnetic field working in a direction
opposite to the direction in which the magnetic field generated by
the induction heating coil 52 works. As a result, the opposite
magnetic filed generated in the shielding member 160 cancels the
magnetic field generated by the coil 52. Accordingly, the same
magnetism-shielding effect as the first to third embodiments can be
obtained.
[0185] Further, the specific forms of each component element
including the arch core 54 or the side cores 56 are not limited to
the ones shown in the figures, and thus, can be suitably
varied.
[0186] The image forming apparatus and particularly, the fixing
unit described so far mainly have the following configuration.
[0187] The image forming apparatus includes An image forming
apparatus includes an image forming section forming a toner image
and transferring the toner image onto a sheet and a fixing unit
including a heating member and a pressure member. The fixing unit
is operable to fix the toner image onto the sheet while nipping and
conveying the sheet between the heating member and the pressure
member. The heating member has a sheet conveyed region that the
sheet passes. The sheet conveyed region is set in accordance with
the size of the sheet being conveyed. The fixing unit further
includes a coil arranged along an outer surface of the heating
member and generating a magnetic field, a fixed core arranged
opposite to the heating member with respect to the coil and forming
a magnetic path, a plurality of movable cores arranged between the
fixed core and the heating member with respect to a direction in
which the coil generates a magnetic field, to form the magnetic
path together with the fixed core, and also arranged along the
sheet conveyed region, a shielding member arranged along an outer
surface of at least one movable core and shielding magnetism, and a
magnetism adjustment unit rotating at least one movable core around
a predetermined axis to switch the position of the shielding member
between a shielding position where the shielding member is
positioned inside the sheet conveyed region to shield the magnetism
and a retracted position where the shielding member is positioned
outside the sheet conveyed region to permit pass of the
magnetism.
[0188] The image forming apparatus having the above configuration
employs the method (external IH) of giving induction heating to the
heating member by a magnetic field generated by the coil to heat
and melt a toner image. Therefore, there is no need to provide any
particular member inside the heating member. Besides, in order to
form a magnetic path for leading a magnetic field generated by the
coil, the fixed core is arranged around the coil, and the plurality
of movable cores are simply arranged between the fixed core and the
heating member, thereby avoiding making the space occupied by the
whole thereof larger.
[0189] Furthermore, the image forming apparatus having the above
configuration is capable of adjusting the generated-heat quantity
of the heating member only by rotating at least one movable core.
Specifically, if the magnetism adjustment unit rotates the movable
core to move the shielding member to the retracted position, a
magnetic field generated by the coil is led to the fixed core and
the movable core, causing the heating member to generate an eddy
current and conducting magnetic induction heating. On the other
hand, if the magnetism adjustment unit rotates the movable core to
move the shielding member to the shielding position, the magnetic
resistance in the magnetic path increases to lower the magnetic
field strength, thereby reducing the generated-heat quantity of the
heating member.
[0190] Moreover, in the image forming apparatus having the above
configuration, there is no need to move the core away from the
heating member in adjusting the generated-heat quantity of the
heating member, thereby saving a space. Besides, there is no need
to provide inside the heating member a core for magnetic induction
or an electrically-conductive member for magnetic field adjustment,
thereby suppressing an increase in the heat capacity and shortening
the warm-up time.
[0191] In addition, in the image forming apparatus having the above
configuration, it is preferable that the shielding member is
provided on the outer surface of each movable core and the
magnetism adjustment unit rotates the plurality of movable cores
individually.
[0192] According to this configuration, the plurality of movable
cores are individually rotated to switch the position of the
shielding member of each movable core independently, thereby
adjusting the generated-heat quantity of the heating member in
accordance with a variety of sheet sizes (sheet conveyed regions).
For example, when the sheet size is minimum, control is executed in
such a way that the shielding member of the movable core outside
the minimum sheet conveyed region with respect to the sheet width
direction is switched to the shielding position, thereby preventing
an excessive temperature rise in the heating member outside the
minimum sheet conveyed region. Besides, if the sheet size is
changed, control is executed in such a way that the shielding
member of the movable core outside the sheet conveyed region in
accordance with the sheet size is switched to the shielding
position, thereby quickly responding to a switch of the sheet size
while certainly preventing an excessive temperature rise in the
heating member outside the sheet conveyed region.
[0193] Furthermore, in the image forming apparatus having the above
configuration, preferably, the magnetism adjustment unit includes a
common rotation unit simultaneously rotating the outer movable
cores arranged at positions corresponding to ends of a maximum
sheet conveyed region set when a sheet having a maximum size is
conveyed, and a plurality of individual rotation units individually
rotating a corresponding one of the other inner movable cores
positioned between the outer movable cores.
[0194] According to this configuration, the common rotation unit
rotates outer movable cores together to switch the respective
shielding members simultaneously to the shielding positions,
thereby screening out magnetism easily on the outermost side of the
sheet conveyed region and quickly responding to a switch of the
sheet size.
[0195] Moreover, in the image forming apparatus having the above
configuration, it is preferable that the outer movable core and the
inner movable core are each a cylindrical core having a through
hole formed along the axis thereof. It is also preferable that the
common rotation unit includes a rotating shaft member fitted in the
through holes of the outer movable cores and fitted loosely in the
through holes of the inner movable cores, and a drive source
rotating the rotating shaft member whereas each of the individual
rotation units includes a rotating roller pressed into contact with
an peripheral surface of the corresponding inner movable core and
undergoing rotation to transmit a friction force to the peripheral
surface, and a drive source rotating the rotating roller.
[0196] In addition, in the image forming apparatus having the above
configuration, it is preferable that the movable cores include a
first movable core arranged inside a minimum sheet conveyed region
set when a sheet having a minimum size is conveyed and a second
movable core arranged outside the minimum sheet conveyed region and
also that the shielding member is provided in not the first movable
core but the second movable core.
[0197] According to this configuration, since the shielding member
is not provided in the movable core arranged inside the minimum
sheet conveyed region, screening out magnetism by the shielding
member is not carried out, thereby constantly transmitting a
magnetic flux to the heating member.
[0198] Furthermore, in the image forming apparatus having the above
configuration, it is preferable that among the plurality of movable
cores, the magnetism adjustment unit rotates a movable core
arranged outside the sheet conveyed region set in accordance with
the size of the sheet to switch the position of the shielding
member of the movable core from the retracted position to the
shielding position.
[0199] According to this configuration, the shielding member of the
movable core inside the sheet conveyed region (within the heated
range) is switched to the retracted position, a magnetic field
generated by the coil passes the fixed core and the movable core,
thereby causing the heating member to generate an eddy current and
conducting magnetic induction heating. On the other hand, when the
magnetism adjustment unit rotates the movable core outside the
minimum sheet conveyed region to move the shielding member to the
shielding position, the magnetic resistance inside the magnetic
path increases to lower the magnetic-field strength, thereby
reducing the generated-heat quantity of the heating member. This
makes it possible to certainly prevent an excessive temperature
rise in the heating member outside the sheet conveyed region.
[0200] Moreover, in the image forming apparatus having the above
configuration, it is preferable that the plurality of movable cores
are formed by dividing a single core into a plurality of cores and
the single core has a through hole of a circular sectional shape
formed along the axis thereof. It is also preferable that the
magnetism adjustment unit includes a shaft member fitted loosely in
the through holes of the movable cores and supporting the movable
cores rotatably, a guide groove formed in an inner peripheral
surface of each movable core, an engagement portion provided in the
shaft member and engageable with the guide groove, and a drive
mechanism driving the shaft member. The shape of the guide groove
is preferably set in such a way that as the shaft member is driven,
the engagement portion moves in the guide groove to rotate the
movable cores.
[0201] The magnetism adjustment unit preferably has the following
specific configuration. The engagement portion is a plurality of
projections provided on an outer peripheral surface of the shaft
member and spaced at a predetermined interval from each other in
the axial direction of the shaft member. The drive mechanism
includes a moving mechanism moving the shaft member in the through
holes in the axial direction of the movable cores and a rotation
mechanism rotating the shaft member in the through holes around the
axis of the shaft member. The guide groove includes an axial groove
formed at the inner peripheral surfaces of the movable cores over
the movable cores in the axial direction of the movable cores, and
a circumferential groove formed at the inner peripheral surface to
extend from the axial groove in the circumferential direction of
the movable core. The axial groove has a shape capable of receiving
the projections. The projections move in the axial groove relative
to the movable cores in the axial direction of the movable cores
when the moving mechanism moves the shaft member. The
circumferential groove has a shape capable of receiving the
projections. The projections move in the circumferential groove
relative to the movable cores in the circumferential direction of
the movable core when the rotation mechanism rotates the shaft
member. When the moving mechanism moves the shaft member in the
axial direction, the projections are switched to a position where
the projections are received in the circumferential groove or a
position where the projections are not received in the
circumferential groove. When the projections are switched to the
position where the projections are received, the rotation of the
shaft member by the rotation mechanism keeps the shielding member
in the retracted position, while when the projections are switched
to the position where the projections are not received, the
rotation of the shaft member by the rotation mechanism switches the
position of the shielding member from the retracted position to the
shielding position.
[0202] The magnetism adjustment unit having the above configuration
is capable of selectively rotating the movable cores individually
only using the moving mechanism and the rotation mechanism, thereby
making it unnecessary to employ a rotation mechanism having a motor
for each movable core to simplify the structure.
[0203] In addition, in the image forming apparatus having the above
configuration, it is preferable that the movable core is a
cylindrical core and, instead of the shielding member, includes a
cut-out portion so formed by cutting off a peripheral part thereof
as to have an arcuate shape in section viewed from the axial
direction. When the projections are switched to the position where
the projections are received in the circumferential groove, the
rotation of the shaft member by the rotation mechanism keeps the
cut-out portion in the retracted position, while when the
projections are switched to the position where the projections are
not received in the circumferential groove, the rotation of the
shaft member by the rotation mechanism switches the cut-out portion
from the retracted position to the shielding position.
[0204] According to the above configuration, when the magnetism
adjustment unit rotates the movable core to switch the cut-out
portion to the retracted position, a magnetic field generated by
the coil passes the fixed core and the movable core, thereby
causing the heating member to generate an eddy current and
conducting magnetic induction heating. On the other hand, when the
magnetism adjustment unit rotates the movable core to switch the
cut-out portion to a resistance position (shielding position), the
magnetic resistance inside the magnetic path increases (an air gap
is substituted for a part of the magnetic path) to lower the
magnetic-field strength, thereby reducing the generated-heat
quantity of the heating member. Likewise in this case, the movable
cores are arranged in the width direction of a sheet, thereby
preventing an excessive temperature rise in accordance with a
variety of sheet sizes. Besides, the individual movable cores are
rotated to switch the cut-out portion to the resistance position,
thereby certainly suppressing the quantity of magnetism passing
outside the sheet conveyed region.
[0205] This application is based on Japanese patent application
serial Nos. 2008-085377 and 2008-170520, filed in Japan Patent
Office on Mar. 28, 2008 and Jun. 30, 2008 respectively, the
contents of which are hereby incorporated by reference.
[0206] Although the present invention has been fully described by
way of example with reference to the accompanied drawings, it is to
be understood that various changes and modifications will be
apparent to those skilled in the art. Therefore, unless otherwise
such changes and modifications depart from the scope of the present
invention hereinafter defined, they should be construed as being
included therein.
* * * * *